Step 10: Measure QT interval for specific diagnoses

The QT interval can be prolonged secondary to metabolic disorders and drug effects. It must be corrected for heart rate since it is rate dependent. The corrected QT interval is calculated using the following formula:

  • QTI corrected = (QTI observed) / (square root of RR interval)

The QTI corrected is often reported with computerized ECG interpretation.

This ECG is from a male patient with familial prolonged QT syndrome. The QTI corrected is approximately 0.52 seconds. Normal QTI corrected: 0.40 seconds for males; 0.44 seconds for females.